Installation for recovering energy from solid fossil fuels, more particularly bituminous coal high in inerts

ABSTRACT

An installation for recovering energy from solid fossil fuels more particularly bituminous coal high in inerts consists of at least one unit in which the solid fuels are converted into gas. A gas-turbine and a steam-turbine are provided to recover the energy from the gases. The gases are freed of dust and desulfurized before the gas turbine. The installation is characterized in that a wet-bottom boiler using the ground fuel is provided with pressure firing, the flue gases therefrom being passed to a gas-gas heat exchanger, the desulfurizing unit and dust-removing unit before being utilized in the gas-turbine.

This application is a divisional application of copending applicationSer. No. 925,708, filed July 18, 1978 and now U.S. Pat. No. 4,255,926,issued Mar. 17, 1981.

The invention relates to an installation for recovering energy fromsolid fossil fuels, more particularly fuels high in inerts andparticularly bituminous coal, the said installation consisting of atleast one unit in which the solid fuels are converted into gas, agas-turbine and steam-turbine being provided to recover electricalenergy from the gases, and the said gases being freed from dust anddesulphurized in the unit before the gas-turbine.

The advantage of such units is that, with a suitable combination of thegas-turbine and steam-turbine processes, they provide higher thermalefficiency than a conventional arrangement of individual processes. Thedust-removal serves to clean the gases to such an extent that althoughthe coal used is high in inerts, the said gases may be fed to thegas-turbine. The desulphurization is carried out at a high pressurelevel and therefore has advantages over known unpressurized flue-gasdesulphurization, in that the desulphurizing units are smaller and havelower losses; in the case of adsorptive desulphurization, the adsorptionlosses are reduced by the increased pressure. For this reason,installations of this kind cause very little pollution.

So-called solid-bed pressure gasification is already known. Ininstallations of this kind, coal, mostly high in inerts, is gasified,i.e. partly burned, with some of the combustion air available, and withsteam, under high pressure. In known installations of this kind, thegasification pressure is about 20 bars. The gasification is carried outin a reactor which produces lean gas at a temperature of between 500°and 600° C., which is cooled and then passes to dust-removing anddesulphurizing units. The cleaned lean gas is burned as fuel gas in aboiler under pressure, the flue-gases from which are used to operate thegas-turbine, while the steam produced in the boiler drives thesteam-turbine.

The pressure-gasification of coal, however, results in a series oflosses. Some of these are due to the relatively large amount of unburnedfuel in the ash removed from the reactor, which has hitherto amounted tomore than 10%. Further losses arise from so-called jacket steam, i.e.evaporation of the cooling water fed to the pressure-gasification unit.Other losses are due to the evaporation of water used in so-calledquenching. Finally, still further losses are due to the fact that anappreciable amount of the water used in washing the gases is neverremoved, but remains in the fuel gas in the form of spray and musttherefore be evaporated in the firing of the boiler.

Other disadvantages associated with the pressure-gasification of coalarise from operating difficulties. During the cooling of the lean gas,tar is condensed and this deposited on the dust which is also present inthe lean gas. This produces a mixture of dust and tar which soon leadsto incrustation and blocking of the various circuits in which it occurs,and from which it therefore has to be removed. If substantial heatlosses are to be avoided, the mixture of dust and tar must be separatedfrom the washing medium in a tar-separator or the like unit, and must bereturned to the gas-producer. This returning of the tar raises problems,since the dustcontaining tar can be pumped only to a limited extent andthis always causes operational problems.

Pressure gasification units for coal hitherto used are also relativelydifficult to control. More particularly, the gas-discharge temperature,calorific value, dust content, and tar content are not very constant,which is attributable mainly to the intermittent supply of coal to thegas-producer. Furthermore, a rapid load increase is impossible if anadequately high calorific value of the fuel gases is required at thesame time. These problems make an adequately sensitive output controlimpossible.

Finally, it should be pointed out that, because of their low calorificvalue, the fuel gases also cause problems during combustion in theboiler, and there are always occasions when combustion has to beassisted with additional fuels, more particularly fuel oil.

Also known is an installation of the type described at the beginninghereof in which the fuel is burned and desulphurized in the fluidizedbed. To this end, the ground fuel and desulphurizer are fed to thefluidized bed and the fuel is burned under pressure, in suspension. Thetransfer of heat to the steam process is carried out within thefluidized bed, whereby the combustion temperature is restricted to about900° C. The flue-gas emerging from the fluidizing chamber, andcontaining large amounts of ash, unburned fuel, and partly chargeddesulphurizer, must then be fed to a gas-turbine. So far there is noknown way of producing a gas clean enough for a machine from these fluegases.

If the approach used is to burn the ground fuel in a fluidizing chamber,this has numerous disadvantages, since considerable amounts of unburnedfuel in the ash must also be expected. One particularly difficultproblem is that the unburned fuel is discharged from the fluidizingchamber with the flow of flue-gas and therefore inevitably reaches thesubsequent dust-removing unit. Separating the ash produced by the fuel,and the partly charged desulphurizer, from the flow of flue-gas presentconsiderable difficulties. Another problem is separating thepartly-charged desulphurizer from the ash in order to condition thedesulphurizer. In addition to this are wear problems at the heatingsurfaces which have to be arranged in the fluidized bed. This wear iscaused mainly by the erosion which inevitably occurs in a fluidized bed.Accurate control of fluidized-bed temperatures also causes problems,since if these temperatures become too high, the ash softens and bakesonto the fluidized bed and heating surfaces, whereas if thefluidized-bed temperature is too low, there is a reduction incombustion, i.e. an increase in fuel losses. Partial carbonization alsooccurs, which in turn leads to tar condensation and to baking onto thefluidized bed and heating surfaces.

It is the purpose of the invention, in the case of installations of thetype described at the beginning hereof, to reduce the losses andoperational difficulties, and to design the installation in such amanner that it may also be used as a peak-load power station.

According to the invention, this purpose is achieved in that awet-bottom boiler, accommodating the ground fuel, is provided withpressure firing, the flue-gases therefrom being passed to desulphurizingand dust-removing units before being turned to account in thegas-turbine.

Since the combustion (partial combustion and post-combustion) is carriedout in one unit, the two procedures may be controlled jointly in themanner required for a peak-load power station, i.e. the output may beincreased and reduced relatively quickly and thus adapted to the load,without incurring heat losses and without assisting the combusionprocess with oil or other additional fuels. Since the combustion of thefuel used takes place in a wet-bottom boiler, this eliminates theoccurrence of tar, dispenses with the equipment needed to handle it, anddoes away with the difficulties associated therewith. The pressures atthe flue-gas end of the wet-bottom boiler are 10 bars, for example,which corresponds approximately to the compression ratio of thegas-turbine.

The main advantages of the new installation are, on the one hand, thatthere is a negligible amount, too small to be measured, of unburned fuelremaining in the slag of the wet-bottom boiler. Furthermore, the thermalefficiency is substantially higher, since there are no losses byevaporation externally of the steam circuit--this has hitherto beenunavoidable in the gasification of fuel. The small amount of water inthe flue-gas also assists in improving the thermal efficiency. Upon thisis based the low condensation point and relatively low waste-gas loss.

Another advantage is that most of the ash is removed from the boiler ingranular form and does not get into the machine gas. This granularmaterial may also be processed for other uses.

According to another characteristic of the invention, the flue-gases maybe removed after the radiation portion of the boiler, and heat-exchangesurfaces are provided for these flue-gases externally of the boiler, thesaid heat-exchange surfaces being used to adjust the flue-gastemperature before the gas-turbine. They also produce the steam which isfed to the steam process. It is also advantageous to burn the fueldirectly in a unit under pressure in the cyclone since, as compared withconventional gasification technology, this dispenses with a number ofadditional units (gas producers), and the combustion unit issubstantially smaller than that required in fluidized-bed technology.

It is also possible to arrange the heat-exchange surfaces, used toadjust the temperature of the flue-gas before the gas-turbine, in theflue-gas desulphurizing unit, or the said heat-exchanger surfaces may belocated thereafter.

The actual desulphurizing may be carried out with metal carbonates oroxides, in which case it is possible to operate with a solid bed inwhich the desulphurizer is in the form of pellets or briquettes. On theother hand, it is also possible to use a fluidized bed, or to inject thedesulphurizer into the desulphurizing chamber in the form of a dry dust.

At the flue-gas end it is possible to locate, after the abovementionedheat-exchange surfaces, a dry flue-gas dust-removing unit at theexisting high pressure level. A unit of this kind may operate withceramic filter-candles or with separator nozzles. If required, a hotcyclone may be installed as a pre-separator before the actualdust-removing unit.

As regards subsequent equipment, the invention has the advantage thatthe heat-exchange surfaces behind the radiating portion of the boilerare substantially smaller than the proposed fluidized bed and erosion istherefore less. Moreover, separation of the desulphurizer from the ashis improved because there is considerably less ash than in thefluidized-bed process. This again is a considerable advantage.

In installations of this kind therefore, the invention reduces lossesdue to unburned fuel in the ash, those due to the quenching water, andoperational difficulties, especially those related to mixtures of tarand dust, moreover the installation is designed in such a manner that itmay be used as a peak load power station.

To this end, according to the invention, combustion (partial combustionand post-combustion) takes place in one part of the installation, whichmeans that only one process has to be controlled. This control may be asrequired for a peak-load power station, i.e. the output may be raisedand lowered relatively quickly, and may thus be adapted to the load,without heat losses and without assisting the combustion process withoil or other additional fuels. Since combustion of the fuel takes placein a wet-bottom boiler, no tar is produced, and this dispenses with theequipment hitherto needed to handle it, and with the problems associatedtherewith. At the flue-gas end of the wet-bottom boiler, the pressuresobtaining are 10 bars, for example, which corresponds approximately tothe compression ratio of the gas-turbine.

An installation of this kind ensures mainly that the amount of unburnedfuel in the slag of the wet-bottom boiler is negligible, even too smallto be measured. The thermal efficiency is considerably higher, sincelosses due to evaporation of quenching water externally of the steamcircuit are very largely eliminated, and these have hitherto beenunavoidable in the gasification of fuels. Finally, it is an advantagethat most of the ash is removed from the boiler in granular form anddoes not get into the machine gas, and this granular material may beprocessed for other uses. Moreover, since the fuel is burned directly ina unit under pressure in the cyclone, several additional units (gasproducers) are eliminated, as compared with conventional gasificationtechnology, and the combustion unit is substantially smaller than thoseused in fluidized-bed technology.

However, it is also a purpose of the invention to provide, ininstallations of the type described hereinbefore, for dust-removing anddesulphurizing to be carried out at relatively low temperatures, and forother detrimental substances also to be removed.

To this end, according to the invention, a gas-gas heat exchanger isarranged after the boiler, where the hot flue-gases are cooled withcold, cleaned flue-gas.

In this case a cyclone may be inserted between the super-charged boilerand the gas-gas heat exchanger for rough-cleaning of the flue-gas. Inthe said heat exchanger, the roughly precleaned flue-gas is cooled bythe flue-gas cleaned in the subsequent installation. According to theinvention, the necessary temperature differential in the gas-gas heatexchanger may be obtained by locating, after the gas-gas heat exchanger,an additional heat exchanger in which the feed-water for the steamprocess is preheated, thus cooling the flue-gas still further. Thisarrangement has the advantage that, in a subsequent gas-washing unitusing water, the flue-gas picks up little or no water, and evaporationlosses are thus kept small. The disadvantage of the arrangement is thatthe steam-circulating process is thereby decarnotized.

Another way of producing the necessary temperature differential is byadiabatically saturating the flue-gas with water in the subsequentwater-wash.

After the flue-gas has been cooled as described above, the gas ispresent saturated with steam at temperatures somewhat above 100° C., atwhich time dust and detrimental substances such as chlorine, fluorine,NO_(x) and SO₂ may be removed therefrom by conventional methods.

The invention has the advantage that the cleaning stages operate at apressure of about 10 atm., and therefore require comparatively verysmall units. Furthermore, the temperature level of the flue-gas in thisinstallation is about 40° C. lower than in known installations, and thisimproves both desulphurization and the removal of other detrimentalsubstances such as chlorine, fluorine and NO_(x). After passing throughthe cleaning unit and the subsequent spray-separator, the cleanedflue-gas is reheated in the gas-gas heat exhanger and fed to agas-turbine. The flue-gas emerging from the gas-turbine passes to awaste-heat boiler where it heats up the feed-water for thesteam-circulating process and is thereby cooled to temperatures of about120° C.

Details, further characteristics, and other advantages of the inventionmay be gathered from the following descriptions of a plurality ofexamples of embodiment of the installation according to the invention,in conjunction with the drawings attached hereto, wherein:

FIG. 1 illustrates a first example of an embodiment of the installationaccording to the invention, with fluidized-bed desulphurization of theflue-gases;

FIG. 2 illustrates an example of an embodiment using a modified(injection) desulphurizing unit;

FIG. 3 illustrates an example of an embodiment of an installationaccording to the invention which makes use of a solid-bed desulphurizingunit;

FIG. 4 illustrates a first example of an embodiment of the installationaccording to the invention having a feed-water preheating stage locatedbetween the gas-gas heat exchanger and the flue gas cleaner.

FIG. 5 illustrates an example of an embodiment in which the necessarytemperature differential for the gas-gas heat exchanger is obtainedmerely by adiabatic saturation of the flue-gas.

Coal high in inerts is fed from a bunker 1 to a grinding unit 2, whencethe ground fuel passes, through a gate 3, to a line 4 running to thecyclone firing of a wet-bottom boiler generally marked 5.

In the drawing, line identification is in accordance with GermanIndustrial Standard 2481.

The flue-gases are removed at 7 and pass, in the installation accordingto FIG. 1 into a fluidized-bed desulphurizer 8. The desulphurizing agentused may be, for example limestone which is fed, through a gate unit 9,to the desulphurizing unit at 10 and 11. The desulphurized flue-gasesleave the fluidized bed at 12 and pass to a cyclone 13, which removescoarse solids from the gas. These solids are removed at 14 and passed toa grading unit 15 which may consist of a plurality of screens. Theoverflow from the screens passes, through a line 16 for furtherprocessing or use, whereas the through-put is removed at 17 and,according to the example of the embodiment illustrated, passes to a gate18. Fine dust reaches this gate through a line 20. The fine-dustseparator (a separator nozzle or filter) is marked 21. The flue-gas fromwhich the coarse dust has been removed is passed to this separator.

The flue-gases leave separator 21 at a temperature of between 800° and900° C. and pass through a line 22 to a gas-turbine 23 following awaste-heat boiler 24. After this boiler, the flue-gases are released tothe outside air at 25.

In the example of embodiment illustrated, provision is made for therelease of the separated fine dust, through gate unit 18, pneumatically,through a line 26, back to line 4. In this way, some of the dust isreturned to wet-bottom boiler 5.

The dust and ash are removed as a fluid from the wet-bottom boiler 5 andpass to a hydraulic ash-removing unit with a granule crusher, marked 27in the drawing. The ash granules are separated at 29 by a gate 28 fromthe water carrying them, and are removed.

Boiler-feed water, which is fed at 30 to waste-heat boiler 24, flowsthrough a line 32 equipped at 34 with a branch communicating with theradiating portion of boiler 5. Steam leaves the boiler 35 and passes, inthe example of embodiment illustrated, through a heat-exchanger 36arranged in fluidized-bed desulphurizer 8. The steam then flows througha line 37 to a steam-turbine unit 38 with intermediate superheater 38afollowed by a condenser 39.

As may be seen, the fossil fuels in the form of coal are fed directly tothe combination unit described above. This also applies to theinstallations illustrated in FIGS. 2 and 3.

The example of embodiment according to FIG. 2 differs from that in FIG.1 mainly in the type of desulphurization to which the flue-gases fromline 7 are subjected. The desulphurizing unit is marked 40 in FIG. 2.The medium used is a dry dust forming desulfurizer which is fed todesulphurizing vessel 41 at various points, through nozzles marked42-44. Again, the desulphurizing medium is fed pneumatically, through agate unit 45, from air-supply line 47 shown in dotted lines, to the saidnozzles.

The sulfur and associated desulphurizer leave vessel 41, through a line48, as a solid and is thereafter passed to the grading unit 15. Thedesulfurizer and associated sulfur leave the grading unit 15 in anoverflow at 16. Some of the desulphurizer is again passed, through aline 49, to the gate, while some is removed at 16a for processing orfurther use. The amount removed is replaced at 16b with freshdesulphurizer.

In the example of embodiment according to FIG. 3 the desulphurizing iscarried out, as fluidized-bed desulphurizing, in a reaction vessel 50.In this case, the desulphurizer may be in the form of pellets orbriquettes and may be fed, through a gate, continuously orintermittently. If the desulphurizer and its accompanying sulphur areremoved from vessel 50, this product again reaches, at 52, the screenoverflow from grading unit 15 and thus passes to line 16, from which theconditioned desulphurizer may be returned, at 53, to the process.

As shown in FIGS. 4 and 5, ground coal 101, containing inert material,is fed under pressure to the cyclone firing of a wet-bottom boilermarked generally 103.

The coal is burned under pressure, thus evaporating water for thesteam-circulating process which is superheated at 104. The flue-gasesemerge at 105, are freed from coarse dust in a cyclone separator 106,and are passed to a gas-gas heat exchanger 107, where the uncleanedflue-gas is cooled to about 300° C. by the cold, cleaned gas.

In the installation according to FIG. 4, the flue-gases pass tofeed-water preheater 108 where the temperature is reduced by another100° C., for example. In the following cleaning stage, the saidflue-gases are saturated with steam which reduces the temperature, inthis example, to about 108° C.

In the installation according to FIG. 5, the flue-gas enters thegas-cleaning unit, immediately after the gas-gas heat exchanger, at atemperature of about 300° C., and is cooled thereto about 118° C. bysaturation with steam. Both in FIG. 4 and FIG. 5 the gas-cleaning unitmay consist, for example, of an ammoniacal water-wash 109 where dust anddetrimental substances such as chlorine, fluorine, and NO_(x) areremoved from the flue-gas. The water-wash is followed by awet-desulphurizing unit 110 and a spray-separator 111. The cleanedflue-gas enters the gas-gas heat exchanger and is heated by theuncleaned flue-gas to the gas-turbine inlet temperature 851° C. forexample. Gas-turbine 112 drives a compressor 113 and a generator 114.Compressor 113 supplies the combustion air required to burn the coalunder pressure. Generator 114 supplies the electrical power. After thegas-turbine, the flue-gas passes, at a temperature of 434° C., forexample, into waste-heat boiler 115, thus preheating the feed-waterneeded for the steam-circulating process and being itself cooled to 120°C., for example.

At 116, the preheated feed-water enters the charged boiler chamber,where it is evaporated, superheated at 104, and passed intosteam-turbine 117. The latter drives a generator 118 which also supplieselectrical power.

In wet-bottom boiler 103, the dust and ash component is drawn off as afluid which passes to a hydraulic ash-removing unit with a crusher forgranular material. This granulated ash can be separated from the watercarrying it and removed.

What is claimed is:
 1. An installation for recovering energy from solidfossil fuels, more particularly, fuels high in inerts, and particularlybituminous coal, said installation comprising;at least one steamgenerating, wet bottom boiler pressure fired with ground fuel forconverting the solid fuel to flue gas; gas turbine means operable by theflue gas for driving electrical generations means; steam turbine meansoperable by the steam from said boiler for driving electrical generationmeans; desulfurizing means interposed between said boiler and said gasturbine; gas-gas heat exchanger means interposed between said wet bottomboiler and gas turbine in which pressurized flue gases emerging fromsaid wet-bottom boiler are cooled by subsequently treated flue gases;dust removing means interposed between said boiler and said gas turbine;first gas-liquid heat exchanger means interposed in the outlet of saidgas turbine means, in which the flue gases emerging from said turbineheat feed water for said boiler; second gas-liquid heat exchanger meansinterposed between said gas-gas heat exchanger and said desulfurizingmeans, in which flue gases emerging from said gas-gas heat exchangermeans preheat feed water for said wet-bottom boiler; and a washing unitinterposed downstream of said second gas-gas heat exchanger for removingdust, SO₂, chlorine, fluorine and NO_(x) from the flue gases, saidcleaned flue gases being reheated in said gas-gas heat exchanger todrive said gas turbine.
 2. An installation according to claim 1including a washing unit interposed downstream of said gas-gas heatexchanger for removing dust, SO₂, chlorine, fluorine and NO_(x) from theflue gases, said cleaned flue gases being reheated in said gas-gas heatexchanger to drive said gas turbine.
 3. An installation according toclaim 1 wherein said dust removing means employs ceramic candle-filtersfor separating dust.
 4. An installation according to claim 1 whereinsaid dust removing means utilizes separating nozzles for separatingdust.